(1) Field of the Invention
The present invention relates to a visible ray-recording hologram material. More particularly, the present invention relates to a plastic type organic photosensitive material, from which a hologram can be prepared by interference exposure using argon ion laser beams.
(2) Description of the Related Art
A hologram is formed by recording the interference wave fronts of coherent light such as laser beams as the refractive index distribution or light absorption (light and shade) distribution, and the hologram is used not only as a stereoscopic photo but also as an optical element in which the wavelength separating capacity, light focussing capacity or incident angle selecting capacity of the hologram is utilized. For example, trials have been made to utilize the hologram as a beam scanner for a bar code reading device or laser printer, a pickup lens of an optical disc memory, or a mixing window for a head-up display.
When these optical elements are prepared by using a hologram, the material for the hologram must have the following properties.
(1) Recording is possible with a cheap and high-power visible ray laser.
Namely, a cheap and high-power laser having a long coherent length, in which the difference of the wavelength from a reproduction light source (He--Ne laser of 633 nm or semiconductor laser of 780 nm is ordinarily used) is small, is desired as the exposure light source. As the laser of this type, there can be mentioned an argon ion laser and an He--Ne laser. The argon laser in which an especially high output is obtained is excellent as the light source for the production of a hologram.
(2) The sensitivity is high.
By shortening the exposure time, the noise can be reduced and the productivity can be improved.
(3) The diffraction efficiency of the hologram is high.
(4) The hologram has excellent moisture resistance and heat resistance.
(5) The material is colorless and transparent.
As the photosensitive material for the hologram recording, there are used a product obtained by subjecting a silver salt used for an ordinary photographic material to a bleaching treatment and gelatin dichromate. The silver salt and gelatin dichromate have a high sensitivity substantially over the entire visible ray region, but are defective in that the hologram-forming treatment is complicated, and since a gelatin film is used as the binder, the hologram has unsatisfactory environmental resistance characteristics such as moisture resistance, heat resistance, and light resistance.
As the material overcoming these problems, there has been proposed a polymeric hologram recording material comprising poly-N-vinylcarbazole (PVCz) rendered photosensitive by an organic halogen compound generating a halogen radical upon absorption of light (see, for example, Japanese Examined Patent Publications Nos. 56-1620 and 55-31453 and Japanese Unexamined Patent Publications Nos. 53-15153 and 54-102140). However, most halogen compounds are generally colorless or have a very light yellow color, and therefore, this polymeric material is used for the production of a hologram by using ultraviolet rays or near-violet rays. It has been long known that vinylcarbazole is photo-polymerized by using a halogen compound (see, for example, Japanese Examined Patent Publication No. 37-16085). Also in this case, the reaction is effected by utilizing ultraviolet rays of a mercury lamp or the like. Of course, a few organic halogen compounds have a sensitivity to visible laser beams, and it is known that recording is possible by an argon ion laser (515 nm) by using carbon tetraiodide (see, for example, Japanese Unexamined Patent Publication No. 53-15153). However, carbon tetraiodide is defective in that it is readily decomposed and the pot life is extremely short because of dark reaction by heat. In fact, a hologram obtained by dissolving 8 g of PVCz in 200 g of chloroform, filtering the solution, adding 0.4 g of carbon tetraiodide to form a coating solution, immediately coating the solution and performing light exposure and development has greatly different characteristics to those of a hologram obtained by coating the coating solution 1 hour after the preparation of the coating solution. Moreover, when this coating solution is allowed to stand for 5 hours (in a dark place at room temperature), gelation is caused and coating is impossible, and it is obvious that the coating solution cannot be put into practical use.
Iodoform having 3 iodine atoms is more stable against heat than carbon tetraiodide, and even if a coating solution prepared in the same manner as described above by using an iodoform is allowed to stand for 1 week after the preparation, the increase of the viscosity is less than 10%. However, iodoform has no substantial absorption of light at about 500 nm (see FIG. 1), and therefore, reproduction of a hologram by rays within this region is difficult.